Referring to FIGS. 1A to 2C, a conventional fluorescent luminous tube, for example, a conventional fluorescent display tube, will be described. Like reference numerals in FIGS. 1A to 2C represent like parts.
FIGS. 1A to 1C show various views of a prior art fluorescent display tube, wherein FIG. 1A is a top view thereof; FIG. 1B is a partial cross sectional perspective view of an anchor thereof; and FIG. 1C is a partial cross sectional perspective view of a support thereof.
As shown in FIG. 1A, the prior art fluorescent display tube includes a substrate 30 made of an insulation material such as a glass or a ceramic, cathode filaments F, cathode electrodes 31 and 32, cathode wirings 311 and 321, an anchor 33 and a support 34 of the filaments F. The anchor 33 has a mounting portion 331, resilient portions 332, filament-mounting portions 333 and upper pieces 334 (FIG. 1B). The support 34 has a mounting portion 341, filament-mounting portions 342 and upper pieces 343 (FIG. 1C).
The cathode electrodes 31 and 32 are made of metallic layers or plates formed of aluminum, for example, and are fixed on the substrate 30, e.g., by an adhesive agent of a fritted glass, and so forth. The mounting portions 331 and 341 of the anchor 33 and the support 34 are fixedly adhered to the cathode electrodes 31 and 32 by welding, respectively. One end portion of each filament F is interposed between a filament-mounting portion 333 of the anchor 33 and an upper piece 334 welded thereon. Similarly, the other end portion of each filament F is fixedly mounted between a filament-mounting portion 342 of the support 34 and an upper piece 343, fixedly welded thereon. Each resilient portion 332 of the anchor 33 exerts a tensile force on a corresponding filament F.
In such fluorescent display tube, the anchor 33 and the support 34 are formed by press working, thereby increasing the manufacturing costs thereof. Further, since they have three-dimensional shapes with a predetermined strength, reduction of their sizes is limited, which in turn restricts the scaling-down or the reduction in the thickness of the fluorescent display tube. Additionally, the fluorescent display tube in FIGS. 1A to 1C requires a complicated mounting process; i.e., mounting the anchor 33 and the support 34 on the cathode electrodes 31 and 32, respectively, and then mounting the filaments F on the anchor 33 and the support 34. Moreover, since the mounting process is carried out by heating welding such as resistance welding, the cathode electrodes 31 and 32 may be damaged in the course of the welding process if they are thin, and in certain cases, a crack may be developed in the substrate 30 due to the difference in the thermal expansion coefficients of the substrate 30 and the cathode electrodes 31 and 32.
Referring to FIGS. 2A and 2B, there are illustrated a plan view and a cross sectional view of another prior art fluorescent display tube, respectively, wherein FIG. 2B is the cross sectional view taken along the line Y—Y in FIG. 2A. FIG. 2C illustrates a temperature profile of a filament.
As shown, reference numerals 351, 352 represent metallic pieces, made of, e.g., aluminum, for welding filaments F to cathode electrodes 31 and 32, respectively; and 361, 362 represent spacers, made of an insulating material, such as a glass, or a metal, for sustaining the filaments F at a predetermined vertical position. Each filament F has a linear portion F1, coiled portions F2 and F3, and end portions F4 and F5.
One end portion F4 of each filament F is interposed between the cathode electrode 31 and a metallic piece 351 welded thereon. Similarly, the other end portion F5 of each filament F is fixedly mounted between the cathode electrode 32 and a metallic piece 352 fixedly welded thereon. Coiled portions F2 and F3 of each filament F exert a tensile force on a corresponding filament F.
The florescent display tube in FIGS. 2A and 2B does not require an anchor and a support, but necessitates the spacers 361 and 362. Further, spaces are needed for accommodating the metallic pieces 351 and 352, the spacers 361 and 362, the coiled portions F2 and F3 thereto. These spaces are the so-called “dead spaces” which cannot be used in displaying an image. Additionally, the coiled portions F2 and F3 of the filaments F waste power without contributing to the display. Also, as in the case of the fluorescent display tube of FIGS. 1A and 1B, when the end portions F4 and F5 of the filaments F are heating-welded, the cathode electrodes 31 and 32 may be damaged by heat or a crack may be developed in the substrate 30, if the cathode electrodes 31 and 32 are thin.
Referring to FIG. 2C, there is illustrated a temperature profile of a filament F. The horizontal axis represents a lengthwise position in the filament F and the vertical axis represents a temperature of the filament F. The heat generated by the filament F is dissipated by the spacers 361 and 362, lowering the temperature in sections P1-P2 of the filament F. The sections P1-P2 are “end cool zones”, where emission of thermal electrons is absent or insufficient due to the low temperature, and therefore do not contribute to the display. The filament zone contributing to the display is a section P2—P2. The filament F has a core wire, made of tungsten, a rhenium and tungsten alloy or the like, coated with a material, such as ternary carbonate, for emitting thermal electrons. The filament F is driven such that the temperature in the section P2—P2 is maintained at 600-650° C.
The length of each of the sections P1-P2 varies depending on the thickness of the filament F and is, for example, about 10 mm if the core wire is 15 μm in diameter.